A check valve of the type referred to above is described in German Pat. No. 1,609,245, and can be utilized in such devices as vacuum waste water conduits. In such prior art valves, the drive force of the opening mechanism must be relatively powerful in order to overcome the constant, oppositely acting force on the diaphragm provided by the vacuum in the line or conduit. Thus, if the drive mechanism is a cylinder-piston unit adapted to be connected to vacuum to provide the opening force, the surface area of the piston must necessarily be larger than the effective surface area of the diaphragm. Thus, such valve units are large and bulky in relation to the cross section of the conduit.
Two patents of possible interest were cited against the German application on which this application is based, viz., U.S. Pat. No. 3,451,423 and German Offenlengungsschrift No. 1,947,396. The former discloses two coaxially interconnected pistons, one of which mechanically engages a valve diaphragm. The latter discloses a valve diaphragm which is acted upon directly by a pressure medium.